Unravelling GLOF Hazards in the Sikkim Himalaya: Simulating Multi‐Lake Flood Scenarios Under Extreme Conditions

In recent decades, the Himalayan region has witnessed accelerated glacier retreat, driven by climate change, leading to the formation and expansion of glacial lakes. These lakes pose significant threats in the form of glacial lake outburst floods (GLOFs), which can be triggered by external events such as rockfalls, ice avalanches, intense precipitation and seismic activity. Previous Himalayan GLOF studies largely focused on single‐lake breaches, whereas the cascading or simultaneous failure of multiple glacial lakes under regionally correlated triggers remains insufficiently explored. Given that many high‐hazard lakes are located within similar hydro‐climatic and seismic zones, understanding worst‐case GLOF scenarios involving simultaneous breaches from multiple lakes requires detailed investigation. This study addresses this critical gap by rigorously quantifying the impacts of multi‐lake GLOF events, considered among the most severe hazards threatening downstream communities. Using two‐dimensional hydrodynamic modelling in HEC‐RAS, we simulate the hydraulic behaviour of GLOFs originating from three highly vulnerable lakes: South Lhonak Lake, Gurudongmar Lake and Shako Cho Lake. The analysis encompasses seven distinct scenarios (S1–S7), ranging from individual lake outbursts to simultaneous breaches of all three lakes. Our findings reveal maximum flow depths ranging from 5 to 20 m, with flow velocities between 1 and 4 m/s. The worst‐case scenario produced a peak discharge of approximately 9400 m 3 /s at Chungthang, a critical downstream settlement. These findings highlight the potentially catastrophic consequences of multi‐lake GLOF events, emphasizing the urgent need for integrated hazard assessment and proactive mitigation planning in vulnerable Himalayan river basins.